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  <a href="http://sudopedia.enjoysudoku.com/Avoidable_Rectangle.html">Avoidable Rectangle - Type 2</a>
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   If <b>digit {4}</b> were entered into <b>cell {3}</b> and <b>digit {5}</b> into <b>cell {2}</b> the four <b>cells {0}, {1}, {2} and {3}</b> would form a 
   <a href="http://sudopedia.enjoysudoku.com/Deadly_Pattern.html">deadly pattern</a> because there are two possibilities to
   place the two digits into the four cells (none of the two solved cells are givens/clues and can therefore be switched for another digit).<br/> 
   This would lead to an invalid puzzle with two solutions (a valid puzzle can have just one solution).<br/>
   To avoid the deadly pattern either <b>cell {3} or {2}</b> must be <b>{6}</b> and none of the cells seen by both cells can contain <b>digit {6}</b> as a candidate. 
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